I'm trying to teach myself how to code little Plugins in Lua but the API to be found on the Wiki seems to outdated. Can anyone show me where to find an up-to-date one or how to create one, if there is any possibility? To me, it would be enough to know which enums / functions / classes exist.

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"Our ideals may never be realised, But they indicate what we are trying to do." Dick Tahta

I assume that you have already looked at the Stickies over at forums.lotro.com
the Lua Scripting (Beta) forum thread -- about 2/3 of the way down the forums list.

There is also a pretty detailed "how to" by Garen:
Writing LoTRO Lua Plugins for Noobs.

The latest update from Narrel is the thread: Riders of Rohan Lua/Skinning updates. (Narrel is the Dev responsible.)

As far as I know he has not posted an the Riders of Rohan update he mentions, only the skinning update. Therefore the most recent API document is here (lotrointerface) in LotRO Tools & Utilities for Developers - the thread: Isengard Lua API Documentation